(1) Field of the Invention
The invention relates to processes for packaging a MEMS microphone, and MEMS packages produced using the method, and more particularly, to a method of packaging a MEMS microphone having good environmental protection and a sufficient back chamber.
(2) Description of the Related Art
Micro-electro-mechanical systems (MEMS) devices are known which convert physical phenomena, such as pressure, acceleration, sound, or light, into electrical signals. Each type of MEMS device interacts with the world in a different way, and demands custom or at least semi-custom packaging solutions. So-called system-in-package techniques attempt to form an entire microsystem—which could include a microprocessor, communications components, actuators and sensors—within a single package. However, packaging of a MEMS device is totally different from packaging an integrated circuit. MEMS devices are categorically different from ICs despite sharing some fundamental processing technologies. Packaging is the biggest challenge for commercializing most MEMS devices. The term “MEMS package” is used in this document to imply a package including at least one MEMS device.
The packaging requirements of a MEMS microphone or acoustic sensor are complex because the devices need to have an open path to receive sound pressure and yet the devices need to be protected from external environmental hazards like particles, dust, corrosive gases and humidity. In addition, the device must have a.back chamber to be functional. A sufficient back chamber is an essential requirement to guarantee the desired performance of any microphone/acoustic sensor. An insufficient back chamber will damp the diaphragm movement and increase the acoustic noise. In considerations of packaging the MEMS microphone, it is also desired to avoid expensive substrate and packaging materials.
For example, FIG. 1A illustrates a low cost package concept. The MEMS microphone/acoustic sensor 102 is mounted on a low cost 2-layer PCB substrate 100. A metal lid 104 has an opening 105. The opening 105 away from the MEMS device can avoid any direct impact from environmental hazards and thus provides a good environmental package protection. However, there is an insufficient back chamber behind the MEMS microphone/acoustic sensor. FIG. 1B illustrates another low cost package concept with a 2-layer PCB and a metal lid 104. The space enclosed by the metal lid and the PCB substrate provides a sufficient back chamber. However, the opening 105 under the MEMS microphone/acoustic sensor 102 directly exposes the diaphragm to external environments, making it susceptible to external hazards.
Some prior art patents have attempted to address these concerns. U.S. Pat. No. 6,781,231 to Minervini describes a method of packaging a MEMS microphone into a house formed by a substrate having at least one conductive layer and a conductive cap. However, this provides an insufficient back chamber, similar to the packaging concept illustrated in FIG. 1A.
U.S. patent application Ser. No. 2002/0102004 A1 to Minervini discloses a MEMS microphone package including an expensive multi-layer PCB to create the required back chamber under the MEMS sensing element.
Co-pending U.S. patent application (AL05-001) Ser. No. 11/182254, filed on Jul. 15, 2005, teaches the methods of using flexible substrates and folding processes to make MEMS packages. However, the MEMS die may be exposed to external environmental hazards using this method. Co-pending U.S. patent application (AL05-005) Ser. No. 11/333579, filed on Jan. 17, 2006, discloses the use of a metal meshed hole for environmental protection, but the flexible substrate and processing costs are still high.